Arsenic contamination of groundwater in Bangladesh is now a well established
fact of life. Preventive measures are imperative. However, before designing
any successful arsenic mitigation plan it is necessary to determine the
extent of the problem, as well as the geologic factors that control the
source and mobility of arsenic in groundwater. Many studies are conducted
and many others are underway to achieve this goal. In April 1997 a World
Bank Fact Finding Mission visited Bangladesh to assess the situation and
to initiate a mitigation programme. As a part of the "Rapid Investigation
Programme" recommended by the World Bank, the UK Department for International
Development (DFID) agreed to finance a project to evaluate the arsenic
contamination. On behalf of the Government of Bangladesh (GOB), DFID appointed
British Geological Survey (BGS) as overall consultants for the study. BGS
in turn appointed Mott McDonald Ltd (MML) to carry out the bulk of the
Phase I work. Recently, MML have submitted a report of Phase I.

I have read the "Executive Summary of the Main Report of Phase I, Groundwater
Studies of Arsenic Contamination in Bangladesh" by BGS and MML for the
GOB. This report is the most comprehensive compilation of pre-existing
data and the results of new analyses on groundwater collected from Bangladesh.
Both BGS and MML deserve a round of applause. They did a great job in summarizing
the extent of the arsenic problem in Bangladesh. I understand that this
is an executive summary of the first of two phase reports, and that a five-volume
report is available for purchase. Although the full report probably has
a detail explanation of all the findings, the executive summary stands
alone and should be internally consistent with the elaborate report. It
is based on this premise that I decided to make a few comments on the summary.
I have organized my comments on various topics in the same order that the
writers used in their table of content. However, I did not make comments
on each topic listed on the table of content.

Review of existing data:Based on existing data it appears that
arsenic contamination is maximum in the southern and northeastern Bangladesh
at a depth of 10 to 100 meters. However, the authors mentioned the spatial
variability and patchy nature of arsenic distribution. According to earlier
reports by other authors and organizations (NIPSOM, DCH, etc.), arsenic
contamination is very high in southwestern districts as well. According
to the report, Arsenic seems to be of geologic origin. Sediments deposited
on delta plain and coastal plain in Bangladesh originate in various parts
of the Himalayas and are carried by rivers that flow through India. To
understand the full extent of the spatial distribution of arsenic and the
geochemical processes that control the transport and sink of arsenic, one
will need to analyze both sediment types and arsenic concentrations in
water for the entire Ganges-Brahmaputra-Meghna watersheds. If arsenic solely
originates by adsorption-desorption of arsenic-rich iron oxyhydroxides
then arsenic must be present in the aquifers of the entire watershed. Is
it the case? We know that arsenic contamination is a problem in some parts
of West Bengal. Did anyone look for arsenic in groundwater in, say, Uttar
Prodesh, Bihar, Assam, Tripura. Geologic processes (such as flooding, arsenic
contamination, etc.) are not bounded by political boundaries. We need to
know the nature of distribution of arsenic at various reaches of the watersheds
- from the place of origin (Himalayas) to the various depositional environments
in India and Bangladesh. As the surface water and groundwater flow from
outside Bangladesh, do the concentrations of arsenic increase or decrease?
What geochemical processes are dominant along the flow path of groundwater?

Collection of existing data: It is mentioned in the summary that
"the groundwaters in regional survey have characteristics typical of reduced
groundwater: high dissolved iron, manganese, and low sulphate concentrations."
Since the data were collected by various organizations, how could the author
possibly know how reduced the groundwater was at the time of collection?
Arsenic is a very redox sensitive element and its mobility and speciations
are very much controlled by pH and Eh of the groundwater. This summary
does not mention the pH and Eh range at the time of collection. These measurements
should be done in the field, not in the lab, using flow-through cell, which
are also used to measure other sensitive field parameters, such as pH,
Temperature, dissolved oxygen, and total dissolved solid. The type of dissolved
iron species are also pH-Eh sensitive and are important in arsenic mobility.
It is not clear from this summary as to what methods or measures were taken
to ensure the quality of these measurements.

Small-scale variability: the special study areas:Approximately
50 wells per thana (Nawabganj, Faridpur, and Laksmipur) were studied in
greater detail to determine the small-scale variability in arsenic contamination.
Lithologic logs were examined to determine the structure and continuity
of aquifers. It is not clear if new wells were dug for this purpose. I
have serious concern about pre-existing lithologic logs in Bangladesh.
Firstly, to my knowledge (I worked for a relevant GOB organization) no
detail lithologic logs are kept by DPHE. A lithologic log described by
engineers or drillting technicians generally do not serve the purpose of
understanding geologic nature of sediments and the depositional environments
that various lithosomes represent. Lithologic logs described by geologists/hydrogeologists
can only be used for this purpose. In addition, privately dug hand-pump
owners do not keep any lithologic logs. They dig their wells based on experience
about the water-bearing sand layer in a locality. Secondly, examination
of pre-existing logs is not adequate to understand small-scale variability
in mineral composition, because those logs were not meant to be used for
such detail analyses. Thirdly, one log per 7 km2 is not adequate for small-scale
variability or arsenic concentration. Several dozens of monitoring wells
need to be placed around a hot spot (both up-gradient and down-gradient
of the flow) of contamination - more like studying a plume from a landfill
and leaking underground storage tank.

The summary also mentions that "not all of the wells in the hot spot
are contaminated, but some are." The question is why? Obviously, there
are geochemical variations in terms of mineral composition, extent of aquifer
vs. aquiclude materials, organic content, pH-Eh, etc., which need to be
determined.

The sedimentary depositions in a delta plain or coastal plain show a
great deal of variability in terms of facies change. A simple layer-cake
approach in analyzing spatial and temporal variability in the aquifer materials
will lead to erroneous conclusions. This perspective seems to be missing
from this summary report. To get a better understanding of the subsurface
geology of Bangladesh, without which it will not be possible to understand
the spatial and temporal variability of Arsenic contamination, a series
of new wells will have to be dug and detail lithologic logs maintained
by professional geologists. Once there are enough geologic control of the
aquifers, then the pre-existing lithologic logs can be used to supplement
the newly acquired data.

Geologic sources of arsenic: According to the summary, a high
proportion of the arsenic in the sediments is present as adsorbed arsenic.
This would not be true of arsenic present in primary minerals such as arsenic-rich
pyrite. Arsenic can be derived from both adsorbed arsenic (e.g. in pH >8
and Eh <-250 mv) and from arseno-pyrite by oxidation. Therefore, it
cannot be concluded as to what is the major mechanism by which arsenic
is being introduced in groundwater in Bangladesh. My understanding is that
depending on Eh-pH conditions and other mineralogic parameters (e.g. amount
of dissolved iron species, sulfate or phosphate present), arsenic mobility
and chemistry varies from place to place and from surface water to aquifers.

The summary mentions about "physical separation of sediments during
their transport and reworking in the delta region has resulted in a separation
of arsenic-rich minerals in fine-grain sediments in lower part of the delta."
According to the summary, "this is likely to be responsible for the greater
contamination in the south and east of Bangladesh". This is probably true,
however, the delta has been prograding over geologic time. As a result,
the areas that are located inland at the present time used to be the lower
reaches of delta in the past. Before any such generalization can be made,
it will be necessary to determine the paleogeographic maps showing locations
of various environments of the delta for various geologic times. To my
knowledge, no such study has been done in great detail.

This summary also makes some comments about the sea-level fluctuations
and its impact on arsenic concentration. According to the summary "all
highly contaminated groundwater occurs in sediments deposited since last
glacial period, i.e. 18,000 years ago.

This is a very naïve statement and lot more need to be understood
(such as, ages of various layers and groundwater, paleogeographic reconstruction
of the delta as mentioned earlier, occurrence of arsenic in incised valley-fill
deposits, etc.). In addition, the location of lower part of the delta must
have been beyond the "Swatch of No Ground" in the Bay of Bengal - not inland
- during the low sea-level stand.

Influence of pumping and irrigation: According to the summary,
"older wells are more likely to be contaminated than recently constructed
ones." Why? No explanation was offered. Also, many older wells are constructed
using metallic pipes. It need to be studied to see if corrosion of the
pipes accelerate or enhance arsenic contamination. It is not known if the
type of pipes (PVC vs. metallic) makes any difference in contamination.

Although it seems that shallow wells are more contaminated than the
deeper ones, it cannot be concluded that all deep aquifers are safe, especially
when reduced conditions (deeper wells must be more reduced than the shallower
ones) control arsenic contamination and mobility.

Solutions?:Based on the extent of arsenic problem in Bangladesh
and based on the complexity of arsenic mobility in groundwater, it appears
to me that it is almost impossible to treat the aquifers in Bangladesh
by any treatment procedure that is currently available. Our only option
is to treat the pumped water at point of entry (POE) or point of use (POU).
Ferric hydroxide, Fe(OH)3, apparently is the best sorbent of arsenic at
low pH<8. Ferric hydroxide, which is very abundant in red soil of, say,
Savar, Madhupur Garh, and the Barind Tract probably can be used in filtering
mechanism. Since arsenic is a redox sensitive element and since our treatment
will have to occur on surface water, any adsorption reaction that is feasible
at high Eh (oxidizing environment) will be the only one that can be used.
Ferric iron (as opposed to ferrous iron) is stable under high Eh.

Conclusions: While I think the BGS and Mott McDonalds' report
is a very good compilation of existing data, it raises more questions pertaining
to their interpretation of geologic nature and its relevance to arsenic
contamination in Bangladesh than it answers. More study need to be carried
out to better understand the spatial-temporal variability and the controlling
geologic factors of arsenic contamination in Bangladesh. This report is
a first step in the right direction. A holistic approach is necessary to
solve the arsenic disaster in Bangladesh. Hopefully, the questions raised
will be addressed or answered by the authority concerned before proceeding
to the second phase of the arsenic study in Bangladesh.